IWA affiliate
Drinking Water Hardness:
Reasons and Criteria for Softening
and Conditioning of Drinking Water
Prepared by:
Kiwa Water Research, AwwaRF, TZW and Veolia
April 2007
Global Water Research Coalition
Alliance House
12 Caxton Street
London SW1H 0QS
United Kingdom
Phone:
+44 207 654 5545
www.globalwaterresearchcoalition.net
Copyright © 2007
by
Global Water Research Coalition
ISBN 987-90-77622-18-6
Global Water Research Coalition
Global cooperation for the generation of water knowledge
GWRC is a non-profit organization that serves as a collaborative mechanism for water
research. The benefits that the GWRC offers its members are water research information
and knowledge. The Coalition focuses on water supply and wastewater issues and
renewable water resources: the urban water cycle.
The members of the GWRC are: the Awwa Research Foundation (US), CRC Water Quality
and Treatment (Australia), EAWAG (Switzerland), Kiwa Water Research (Netherlands),
PUB (Singapore), Suez Environment – CIRSEE (France), Stowa - Foundation for Applied
Water Research (Netherlands), DVGW-TZW Water Technology Center (Germany), UK
Water Industry Research (UK), Veolia- Anjou Recherché (France), Water Environment
Research Foundation (US), Water Research Commission (South Africa), WaterReuse
Foundation (US), and the Water Services Association of Australia.
These organizations have national research programs addressing different parts of the
water cycle. They provide the impetus, credibility, and funding for the GWRC. Each
member brings a unique set of skills and knowledge to the Coalition. Through its member
organizations GWRC represents the interests and needs of 500 million consumers.
GWRC was officially formed in April 2002 with the signing of a partnership agreement at
the International Water Association 3rd World Water Congress in Melbourne. A
partnership agreement was signed with the U.S. Environmental Protection Agency in July
2003. GWRC is affiliated with the International Water Association (IWA).
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Disclaimer
This study was jointly funded by GWRC members. GWRC and its members assume no
responsibility for the content of the research study reported in this publication or for the
opinion or statements of fact expressed in the report. The mention of trade names for
commercial products does not represent or imply the approval or endorsement of GWRC
and its members. This report is presented solely for informational purposes.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Preface
In response to requests from several member states to provide guidance on the possible
long term health effects of desalination for production of drinking water, the World
Health Organisation in November 2003 initiated a process to review the potential health
effects of nutrients in drinking water. This was the starting point for renewed discussions
on possible health effects of calcium and magnesium in drinking water.
This issue was of importance not only for those water suppliers which utilise desalination
processes, but also for the broader international water industry. The GWRC adopted this
issue and established a GWRC task force to review the epidemiological evidence and to
describe the reasons and criteria for softening. This resulted in presentations at the WHO
Conference in Baltimore (April 2006), participation in and contribution to the adjacent
WHO Expert Consultation in Washington (April 2006), publications in Water 21
(February 2007) and the publication of two reports.
This report describes the reasons and criteria for softening and conditioning of drinking
water. It was prepared by Margreet Mons (Kiwa Water Research), Hans van Dijk (Kiwa
Water Research), Dominique Gatel (Veolia), Sebastian Hesse (TZW), Jan Hofman (Kiwa
Water Research), My-Linh Nguyen (AwwaRF) and Nellie Slaats (Kiwa Water Research).
Valuable comments were received from EPA (Michael Schock).
A Second report entitled ‘Evaluating the epidemiological evidence on the effects of
Calcium and Magnesium in Drinking Water on Cardiovascular Disease Rates’ has been
prepared by Martha Sinclair (CRC for Water Quality and Treatment) and Olivier
Schlosser (CIRSEE-Suez Environnement) and is published as GWRC report as well.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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Summary
In 2003 the World Health Organization assembled to address a number of questions
relating to the nutrient composition of drinking water and the possibility that drinking
water could in some circumstances contribute to total dietary nutrition. As follow up of
this meeting a WHO report was published, which stated recommendations on the
minimum calcium and magnesium content of drinking water (WHO, 2006a).While the
WHO review was initiated in the context of desalinated water, it is evident that any health
based guideline value for minimum levels of calcium and magnesium should also
logically be applied to drinking water from conventional sources. This issue is therefore of
importance not only for those water suppliers which utilise desalination processes, but
also for the broader international water industry. As a consequence the question arises
whether these requirements should apply to softened water as well.
This report describes reasons and criteria for softening and conditioning. It clarifies that in
addition to the nutritional aspects of calcium and magnesium in water other healthrelated and environmental aspects need to be considered as well. Both very soft and very
hard water can interact with piping materials which negatively impact the water quality
and the integrity of the piping system. It is therefore of importance that in the discussion
on health effects of drinking water hardness, technology considerations of drinking water
distribution, including the optimal composition of piped drinking water to prevent
corrosion and scaling should be taken into account as well.
These aspects have been presented at the WHO symposium ‘Health Aspects of calcium
and magnesium in drinking water’ in Baltimore (24-26 April 2006) and during the WHO
Expert Consultation in Washington (27-28 April). They now have been included in the
WHO document ‘Meeting of Experts on the Possible Protective Effects of Hard Water
Against Cardiovascular Disease’ which was released in December 2006 (WHO, 2006b).
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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Contents
Global Water Research Coalition
1
Preface
3
Summary
5
Contents
7
1
Introduction
11
2
Fundamentals of drinking water hardness
13
3
Reasons for softening
17
4
Hardness levels and treatment practices within GWRC
27
2.1
2.2
2.2.1
2.2.2
2.2.3
2.2.4
3.1
3.2
3.2.1
3.2.2
3.3
3.3.1
3.3.2
3.3.3
3.3.4
3.3.5
3.3.6
3.4
3.5
4.1
4.2
4.2.1
4.2.2
4.2.3
4.2.4
4.2.5
4.3
Origin of hardness
Definitions & units
Hard vs. soft water
Temporary vs. permanent hardness
Units of hardness
Scaling potential
Introduction
Public Health
Release of lead and copper
Home softening units
Environment
Introduction
Copper concentration sewage water
Emission of detergents
Emission of phosphates
Salt pollution
Energy consumption
Consumer comfort
Economics
13
14
14
14
15
15
17
17
17
19
20
20
20
22
22
23
23
23
24
Introduction
Australia
In general
Hardness Levels in Australian Cities and Towns
Technologies Currently used for Hardness Removal
Reasons for softening in Australia
Proposed Plans for treatment
France
27
27
27
27
28
28
29
29
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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4.4
4.5
4.6
4.7
4.8
4.9
4.9.1
4.9.2
Germany
The Netherlands
South Africa
Switzerland
United Kingdom
United States
Hardness removal
AWWA’s survey of drinking water treatment in the US
32
32
34
34
35
36
37
38
5
Reasons for conditioning
41
6
Optimal composition of drinking water
49
7
Literature
51
5.1
5.2
5.3
5.3.1
5.3.2
5.4
5.4.1
5.4.2
5.5
5.6
5.7
General
Asbestos cement/concrete
Dutcile iron, steel and galvanised steel
Ductile iron and steel
Galvanised steel
Copper
General corrosion
Pitting corrosion
Lead
Copper alloys
Conditioning
41
41
43
43
43
44
44
45
46
46
47
Annex I. Classes of hardness in GWRC countries
55
Annex II. Other indicators for scaling potential
57
Annex III. Treatment technologies
59
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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1 Introduction
Desalination of sea water as a source for drinking water is rapidly increasing worldwide.
An exponential increase of total seawater desalination capacity to 50 million m /d in 2007
has been predicted (Wangnick, 2002). More recently, a market analysis by Global Water
Intelligence (WDR, 2006) foresees a further growth of seawater desalination of 12 %
annually until 2010. The growth will mainly take place in the Gulf region, Spain and
Algeria. After 2010 the main growth is predicted in China and India and will further gain
to 15 % in 2015. As a result, the World Health Organization (WHO) was asked to develop
guidance for desalination. During the process of developing such guidance, questions
about possible health effects of minerals in drinking water were addressed as well.
3
The discussion on the possible health effects of the mineral composition of drinking water
is not new as since the late 1950’s many studies on this topic have been published. The
discussion has been mainly focused on the presence of calcium and magnesium in water
and the incidence of cardiovascular diseases. For a long time, the WHO maintained that
insufficient information was available to draw a conclusion, but the topic was discussed
with renewed interest during a WHO Expert Meeting in Rome in November 2003. As a
follow up to this meeting the WHO published a report highlighting the several aspects of
this issue and the current view on the available information. In this report, minimum
requirements for calcium and magnesium in drinking water were mentioned for the first
time (WHO, 2006).
While the WHO review was initiated in the context of desalinated water, it is evident that
any health-based guideline value for minimum levels of calcium and/or magnesium in
drinking water should also logically be applied to drinking water from conventional
sources. This issue is therefore of importance not only for those water suppliers which
utilise desalination processes, but also for the broader international water industry. The
Global Water Research Coalition as representative of many water related research
institutes worldwide aims to actively participate in and contribute with scientific
information to the discussion. It has the opinion that for a clear discussion drinking water
technology should be highlighted as well. This review was undertaken by the Global
Water Research Coalition to describe the reasons and criteria for softening and
conditioning of public drinking water and the technological background of the processes.
Separate reviews were undertaken on the quality of the epidemiological evidence on this
issue and the biochemical/biomedical aspects.
These documents have contributed to the discussion at the WHO symposium ‘Health
Aspects of calcium and magnesium in drinking water’ in Baltimore (24-26 April 2006) and
the WHO Expert Consultation in Washington (27-28 April) following to the symposium.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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2 Fundamentals of drinking water hardness
2.1 Origin of hardness
Groundwater normally remains in the subsoil for hundreds or thousands of years before
it is abstracted. As a result of mineral weathering processes, carbon dioxide (CO ) is
present in the water. If calcium carbonate (CaCO is present in the subsoil this will be
dissolved in the acidic environment caused by the carbon dioxide. Due to the long
residence time in the subsoil groundwater will often be in chemical equilibrium (calcium
carbonic acid equilibrium). When the water is abstracted it comes into contact with the
atmosphere. The excess carbon dioxide will then disappear and the pH will rise until
equilibrium with atmospheric carbon dioxide is reached. Because the pH rises, the
carbonic acid equilibrium will shift towards the bicarbonate (HCO ) side. This results in
precipitation of calcium carbonate (scaling). When the water is heated this effect will
become stronger because, unlike other salts, the solubility of calcium carbonate decreases
at higher temperatures.
2
3)
3
-
The calcium carbonic acid equilibrium is described below:
CaCO + CO + H O ↔ Ca + 2 HCO
3
2
2+
2
3
-
The extent of sub saturation or super saturation of calcium carbonate is described by the
Langelier Saturation index (SI).
[Ca ][CO ]
2+
SI =
log (
.
Ks
2−
3
)
=
pH − pH
s
K = solubility product of calcium carbonate = 0.99*10 mol /l at 25 °C.
[Ca ] = molar concentration of calcium
[CO ] = molar concentration of carbonate
pHs= equilibrium pH of water with equal concentration Ca and HCO
-8
s
2
2
2+
3
2-
2+
3
-
If the saturation index (SI) is greater than zero (SI > 0), the water will be supersaturated,
and calcium carbonate will precipitate (scaling). If the SI value is less than zero (SI < 0),
the water will be sub saturated (aggressive) and dissolve calcium carbonate and
cementous (pipeline) materials (corrosion).
The calcium carbonic acid equilibrium can be visually described by a Ca-pH relationship,
under the simplification that all Ca originates from CaCO (Tillmans assumption) (Figure
1). Above the equilibrium line, the water is supersaturated with calcium carbonate (SI>0)
and will have scale forming tendency. Below the equilibrium line the water is sub
saturated with calcium carbonate (and supersaturated with carbon dioxide) (SI<0) and
will have aggressive properties towards CaCO and cementous materials. Figure 2.1 also
3
3
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
shows that hard water in calcium carbonic acid equilibrium will-have a lower
2+ pH than
CaCO
equilibrium
[HCO
]
=
2
[Ca
]
3
3
soft water in calcium carbonic acid equilibrium.
9,0
8,5
Scaling
8,0
7,5
Hp
7,0
6,5
Aggressive
6,0
5,5
5,0
0,00
0,50
1,00
1,50
2,00
Ca [mmol/l]
2,50
3,00
3,50
4,00
Figure 2.1. Ca-pH relation in equilibrium according to Tillmans conditions ([HCO3-]= 2[Ca2+]).
2.2 Definitions & units
2.2.1 Hard vs. soft water
Whether water is considered as hard or not depends largely on the reference situation in a
country. The hardness classes for several GWRC countries are presented in Figure 2.2. For
more detailed information per country see Annex I.
Hardness classes
Sw itzerland
ryt
nu
oC
very soft
France
very soft
US2
soft
US1
soft
soft
average
moderately hard
Germany
very soft
0
very hard
hard
very hard
class 2
soft
very hard
hard
class 1
Netherlands
very hard
hard
hard
moderately hard
softslightly hardmoderat ely hard
fairly hard
fairly soft
1
fairly hard
2
class 3
class 4
hard
3 mmol/L
very hard
4
5
Figure 2.2. Classes of hardness in GWRC countries. (For additional information see Annex I)
2.2.2 Temporary vs. permanent hardness
In a chemical sense hardness is defined as the sum of the calcium and magnesium content
of drinking water. Therefore, magnesium contributes to total hardness as well. Increasing
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
6
pH causes the precipitation of calcium carbonate. This removes the calcium from the
water, and so removes a portion of the hardness. Therefore, hardness that can be removed
as CaCO is called temporary or carbonate hardness. Other salts such as calcium chloride
(CaCl ), calcium sulphate (CaSO ), magnesium chloride (MgCl ) or magnesium sulphate
(MgSO ) are more soluble and will remain in the water after the calcium carbonate is
precipitated. This is called the permanent hardness.
3
2
4
2
4
2.2.3 Units of hardness
Several units can be used to express drinking water hardness. The relationship among the
units are described in Table 1.
Table 1.1 Units and conversion factors for water hardness
1 mmol/L
1 °D
1 °F
1 meq/L
1 mg CaCO /L
1 gpg
3
1
mmol/L
1
0.18
0.10
0.50
.01
0.17
°D
5.6
1
0.56
2.8
0.056
0.95
°F
10.0
1.78
1
5.0
0.1
1.7
meq/L
2.00
0.36
0.20
1
0.02
0.34
mg CaCO /l
100
17.8
10
50
1
17
3
gpg
5.9
1.04
0.59
2.94
0.059
1
1
gpg = grain per gallon
2.2.4 Scaling potential
The relationship between water composition and precipitation is more complex than
generally assumed. Drinking water hardness is an important parameter in this, but not
the only one. Other factors such as ionic strength and the presence and composition of
organic matter can influence the solubility of calcium. It has been observed that
precipitation of calcium carbonate can also occur in very soft water. In addition, it has
been observed that waters that seem to have an almost identical composition can behave
differently with respect to the extent of precipitation.
Therefore, in the Netherlands, other methods have been developed to measure the
precipitation potential of drinking water. It has been shown, for example that the analysis
of the amount of precipitated calcium carbonate under field circumstances (PACC) shows
a better relation with the scaling effects in every day life than the total hardness (Brink et
al., 1997). More information on these methods can be found in Annex II.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
3 Reasons for softening
3.1 Introduction
Softening of drinking water has been a topic of study in many countries. In several
countries, softening is (sometimes) applied centrally (e.g. the Netherlands, Germany,
Belgium, France, USA). In other countries, the use of point-of-use softeners at the
individual residences is the most applied way of softening.
For several softening processes an obvious reason in favour for softening deals with
aesthetics and comfort. Depending on the softening process other advantages can be
obtained as well (public health, environment, etc.). These other advantages will be mainly
obtained for softening processes going along with an increase of pH and decrease in total
inorganic carbon content (TIC).
This chapter will highlight the advantages of the latter softening processes. Information
on softening techniques can be found in Annex III.
3.2 Public Health
From a public health point of view, softening can have several advantages. These have to
do with:
release of lead and copper
use of home softening units
3.2.1 Release of lead and copper
Metal solving properties of drinking water are influenced by several factors, but the pH
value plays an important role for both copper and lead (Van den Hoven and Van Eekeren,
1988) (see also chapter 4). At lower pH values, metal dissolving properties will be higher
than at elevated pH levels (see Fig. 3.1). Hard water with a high calcium carbonate
content, has a low pH value (see Fig 2.1) and distribution of this low pH water through
lead or copper pipes will result in increased concentrations of these metals in water and
increased exposure of consumers.
Increasing the pH value will reduce metal dissolving properties and will aid in
maintaining the heavy metal concentrations within regulatory boundaries (see table 3.1).
However, before the pH value can be increased removal of calcium carbonate is needed as
at higher pH levels the water will become supersaturated with respect to calcium
carbonate and scaling problems will occur (see Fig. 2.1). Removal of calcium carbonate by
central softening creates the opportunities for increasing the pH value e.g. by adding a
base. Metal solvency will then decrease and exposure of consumers to metals via drinking
water will be reduced. Metal solvency can also be reduced by applying corrosion
inhibitors (see also chapter 5).
Source water quality will determine what softening technology is best to be used.
Softening technologies that use ion-exchange or add soda for example, may increase the
sodium content of drinking water. This is also a health consideration that needs to be
taken into account.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Figure 3.1. Effect of pH on copper concentrations in drinking water (Van den Hoven
et al.
, 1995)
.
Figure 3.2. Average copper concentrations in installation with softening (black marks) and without
softening (open marks) (Becker , 1998).
et al.
Figure 3.2 shows that copper concentrations are lower in installations with central
softening (which resulted in removal of calcium carbonate and pH correction). This study
started with new copper pipes. The effects of removal of calcium carbonate and pH
adjustment on the copper concentrations were observed for both newly installed pipes
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
('Neuinstallation', left side of figure 3.2) and for pipes that had been in use for about 1
year. Even in pipes that had been in use for 300 days, the softening process (removal of
calcium carbonate combined with pH adjustment) ('Umstellung') resulted in a decrease in
copper concentration in these 'old' pipes ('Altinstallation').
Table 3.1. Drinking water standards for heavy metals
Heavy metal
Cadmium (µg/L)
Copper (mg/L)
Lead (µg/L)
Nickel (µg/L)
Zinc (mg/L)
EU
WHO
NL
D
F
UK
USA
A
5
2
10
20
-
3
2
10
20
-
5
2
10
20
3
5
2
25
3
-
5
2
25
20
5
5
2
25
20
-
5
1.3
15
5
2
2
10
20
3
3
3
1
3
1
2
1 = Lead and copper are regulated by a Treatment Technique that requires systems to control the
corrosiveness of their water. If more than 10% of tap water samples exceed the action level, water systems
must take additional steps. For copper, the action level is 1.3 mg/L, and for lead the action level is 0.015
mg/L. If the action level is exceeded by 10% or more of selected tap water samples, utilities are required to
respond with corrosion control treatment and source water management strategies.
2 = non-enforceable secondary maximum contaminant level (SMCL). SMCL is used by the USEPA as a
guideline for evaluating drinking water quality.
3 = after 2013: 10 µg/L
The gains of softening in the reduction of exposure to heavy metals also depend on the
extent of use of certain types of materials for drinking water pipes (copper, lead etc.). For
instance, in the past lead has been applied very often, especially in the inner city of many
older cities. However, some countries (in the E.U.) have phased out the use of lead as a
result of the lowered standard for lead (10 µg/l). This does not necessarily mean that all
lead containing materials have been removed as domestic installations in older houses
may still contain lead. Usually it is the responsibility of the house owner to replace this.
3.2.2 Home softening units
Many consumers appreciate softer water (see also Paragraph 3.4). If no central softening is
applied, consumers may chose to buy their own point-of-use softening units. These units
can be useful in reducing total hardness. However, the risk exists for improper use (e.g.,
installation, replacement, etc.) which could negatively impact the water quality.
Furthermore there is no quality control on the water treated by these softening devices
and its metal dissolving properties. In addition, unregulated plastics and other materials
might be used in these units, which may contribute as a nutrition source for microorganisms and the possibility for migration of unwanted substances to drinking water.
Therefore, it can be argued that central softening at water treatment facilities would
eliminate the need for fitting of private water softening units and prevent issues
associated with the use of these units.
There are several techniques applied in point-of-use units. These can be divided in three
groups:
ion exchange
membrane filtration
physical water treatment
-
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Ion exchange units consist of a resin that exchanges calcium ions with sodium ions. When
the resin is filled with calcium ions, it has to be regenerated thereby replacing the calcium
ions with sodium ions again. Applying ion exchange sodium is added to the water when
removing hardness. This will result in an increased salt content of water. People on
restricted salt diets for health reasons should account for additional salt intake via their
drinking water when using point-of-entry ion exchange units. For more information on
ion exchange see Annex III.
During membrane filtration the water is processed under high pressure through a
membrane unit with specified pore size. For most membrane softening systems,
nanofiltration and/or reversed osmosis are applied. Membrane filtration units will
remove most minerals including calcium and magnesium. Many private Point of Use
membrane units often operate with low recoveries. This means that only a fraction of the
feed water passes through the membrane as softened water, while the remaining part is
wasted. For more information on membrane filtration see Annex III.
Physical water treatment units are not really softeners in the sense of removing hardness.
They are designed to change the mineral structure of calcium by applying magnets. This
should prevent scaling deposits. The underlying principles have never been clearly
elucidated and the effectiveness of this method has been questioned (Hillenbrand et al.,
2004).
Hillenbrand et al. (2004) conducted a questionnaire on the use of point-of-use units in the
community of Eichstetten, Germany. In this region, drinking water hardness is rather
high (> 3.8 mmol/L). The results of the questionnaire showed that in this community
about 42% of the 3200 inhabitants used some kind of point-of-use unit for softening their
drinking water. Of these units about 50% were ion exchangers.
Finally, it should be noted that many dish washing machines contain small ion
exchangers to soften the water. In essence, they can be seen as point-of-use equipment as
well. They are automatically regenerated by sodium chloride.
3.3 Environment
3.3.1 Introduction
There are several environmental benefits resulting from central softening. These have to
do with:
copper concentration of sewage water;
emission of detergents;
emission of phosphates;
use & emission of regeneration salts; and
energy consumption.
-
3.3.2 Copper concentration sewage water
A lower metal content of drinking water (see 3.2.1) is also an advantage from the
environmental point of view as it brings about a reduction in the copper content of both
sewage treatment plant effluent and sludge in which it is concentrated. This can be
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
reached by applying central softening processes as mentioned in par. 3.1. Again, corrosion
inhibitors can be used as well to decrease the metal content (see also chapter 5).
In general the total copper emission to waste water is determined for a large part by
emission from drinking water pipelines. Figure 3.3 shows the contribution of several
pathways to the total emission of copper to the environment in the Netherlands, where
drinking water pipes are incorporated in 'household' and 'office buildings'.
Figure 3.3. Copper emissions to the environment in the Netherlands in 2005 (Roovaart, pers.
comm. 2005).
A study of the federal environment agency in Germany also showed that drinking water
distribution systems contribute significantly to the emission of copper towards surface
water (Fig. 3.4).
,y
na
m
re
G
in a
no /t
is
is
m
er
ep
po
c
1.000
800
600
400
200
0
drinking
water
roof
motor
vehicle
catenary pesticides
(Hillenbrand et al., 2005)
Figure 3.4. Emission pathways for copper to the environment in Germany (Hillenbrand, 2005).
Sewage sludge
Depending on the country sewage sludge might be used in agriculture. An excessive
copper content makes this sludge unsuitable for use in agriculture
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
In Germany, approximately 50 % of the sewage sludge is used in agriculture – limited by
the maximum allowed content of copper of 800 mg/kg dry weight. The other part of the
sewage sludge is increasingly being disposed by incineration.
In the United States, land application of sewage sludge (for agriculture or other purposes)
is regulated by the US EPA under the Federal Standards for the Use or Disposal of
Sewage Sludge (40 CFR Part 503, 1994). The quality of the sludge being applied to the
land is regulated based on (1) the presence of pollutants, (2) the presence of pathogens,
and (3) the attractiveness of the sludge. Copper is one of the ten pollutants being
regulated under this rule. The USEPA has set the following limit for copper in
determining the quality of the sewage sludge (by concentration limits and loading rates):
1. Ceiling concentration at 4300 mg copper per kg dry weight
2. Average monthly copper concentration at 1500 mg/kg dry weight
3. Cumulative copper loading rates of 1500 kg per hectare dry weight
4. Annual copper loading rate of 75 kg per hectare per 365-day period dry weight.
In the Netherlands, the application of sewage sludge in agriculture has been forbidden
since 1985.
Ecotoxicity
The remaining copper will be discharged with sewage treatment effluent to surface water,
which may lead to pollution. As copper is toxic for several aquatic species, contamination
of surface water will lead to adverse ecotoxicological effects (RIVM, 1989).
A lower metal content in drinking water will also decrease the concentration in sewage
sludge, sewage effluent and surface water.
3.3.3 Emission of detergents
During laundry washing with hard water, more detergents are needed for the same
washing result (removal of stains). This is illustrated by Table 3.2, which represents the
instructions on washing powder. Removal of hardness can lead to a reduction in the use
of detergents and a reduced emission of these substances to the environment.
Table 3.2. Dosage instructions washing powder.
4/5 kg
soft water
medium
hard water
slightly dirty
65 mL
95 mL
125 mL
dirty
95 mL
125 mL
155 mL
125 mL = 78 g
extremely dirty
155 mL
190 mL
220 mL
3.3.4 Emission of phosphates
In several countries the drinking water industry applies corrosion inhibitors as one of the
strategies for corrosion control. Some of the corrosion inhibitors often used in drinking
water treatment are inorganic phosphates including polyphosphates, orthophosphate,
zinc phosphates, etc.. This process is applied in several countries, such as Great Britain,
France, the United States and Germany. In Germany the addition of phosphates is
permitted up to a concentration of 5 g/m ³ as a P2O5 as well as silicates to maximum 15
g/m ³ SiO2. In the Netherlands addition of phosphates and other inhibitors is not
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
permitted. Phosphates used for corrosion control will end up in sewage water and enter
the surface water afterwards, thereby contributing to eutrophication.
However, in some situations dosing of phosphate containing corrosion inhibitors is
chosen as means to control corrosion because of the regulatory limit imposed by
trihalomethane formation. For many North American systems possibilities for pH
adjustment to sufficiently high values for corrosion control are limited when there are
concerns with disinfection by-product formation. In these situations reducing by-product
formation will naturally receive a higher priority that the emission of corrosion inhibitors.
3.3.5 Salt pollution
When central softening is applied, private water softeners would no longer be needed.
Many point-of-use softening units use ion exchange as method for softening. As such
devices are often regenerated by means of salt solution, fewer of them in use would lead
to a reduced environmental salt load. Additionally, salts applied in tablets used for dish
washing machines could be phased out.
3.3.6 Energy consumption
Especially for central heating equipment and water cookers scaling will result in increased
energy consumption and therefore CO2 emission.
Hillenbrand et al. (2004) estimated that adverse effects can take place at hardness levels of
1 mmol/l and above. They calculated that energy loss up to 10% could occur. Merkel
(1998) even calculated a 50% increase in energy costs for heating at a hardness of 4
mmol/L compared to 2 mmol/L.
3.4 Consumer comfort
The effects caused by hard water results in discomfort for the consumer. Improved ease of
use of drinking water is something which will be experienced by the consumer as a direct
benefit of softening. Softer water will lead to a decrease in the stiffness and haze in
washed clothes, and fewer marks will be left on plants, glassware or bathroom fittings.
The film on tea made of hard water will not be present after softening anymore, and the
foaming properties of shampoo and shower gel will improve as well. With softened water
the consumer will have to contend less with problems arising out of lime scales in pipes,
shower heads and hot water equipment.
In the Netherlands one of the water companies has conducted a questionnaire study on
the relation between drinking water hardness and consumer discomfort (WML, 2005).
Questions were asked regarding perceived hardness of water, discomfort by scaling
deposits, importance of softening and willingness to pay for softer water. In addition the
number of times that scaling had to be removed from coffee machines, perlators at taps
and water cookers was registered. Figures 3.5 and 3.6 show that the number of descaling
activities was highest in regions with the highest hardness (WML, 2005).
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April 2007
Figure 3.5. Hardness in the Dutch province
Limburg (expressed in PACC90)
Figure 3.6. Number of descaling activities
3.5 Economics
The economic and financial benefit of softer water for the consumer will be expressed in
reduced consumption of washing materials and a reduction in energy consumption in the
heating of drinking water. In addition to this, less maintenance will be required by
heating equipment due to the reduction in lime deposit, and purchase of a private water
softener is rendered unnecessary.
For drinking water companies removal of calcium carbonate accompanied with pH
correction during central softening will result in less corrosion of their infrastructure.
Distribution pipes will have a longer life-time and renewal will be less frequently.
Merkel (1998) has conducted a detailed study on the possibilities of cost saving by
softening for the German situation. He concluded the following saving possibilities for a
family consisting of 4 persons:
Heated water treatment:
Washing:
Dish washing:
Personal hygiene:
50 euro/yr
25 euro/yr
18 euro/yr
1 euro/yr
Total:
140 euro/yr
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April 2007
Total savings will depend on the individual situation (e.g. type of heating equipment,
type of detergent used) and on the situation whether a point-of-use softening unit is used
or not. If a point-of-use softening unit is used savings were calculated up to 200 euro/yr.
Extra costs for central softening were calculated to be about 25 euro/family per year. This
results in a benefit of about 175 euro/yr maximum. The results were calculated for the
situation were water is softened from class 4 (> 3.8 mmol/L) to class 2 (1.3-2.5 mmol/L).
These figures are in line with the calculations made by Hillenbrand et al. (2004). They
calculated the benefits of central softening to be 0.5-0.6 €/m3. For households already
using a point-of-use unit the benefits were calculated to be 0.6-0.9 €/m3. The costs of
central softening were calculated to be between 0.1-0.5 €/m3, resulting in an overall
benefit up to 0.8 €/m3.
Assuming an average water use of 150 m3/year/household this will result in a benefit up
to 120 €.
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4 Hardness levels and treatment practices within
GWRC
4.1 Introduction
Large differences exist between the GWRC countries in the way they handle hard water
and softening. The situation in the GWRC countries will be discussed in the following
paragraphs. Information regarding the amount of water being softened or the number of
softening plants is not always easily available. Therefore for some countries more detailed
information is presented than for others.
4.2 Australia
4.2.1 In general
Australia is a country of 20 million people with approximately 75% living in coastal urban
zones. The responsibility for the water management is largely with the states, and is quite
different from state to state. Due to a highly variable and in general very low rainfall,
major cities are very reliant on large dams for water supplies. These dams are fed from
surface water sources and comprise 98.5% of all supplied drinking water. Only 1.5% is
supplied from groundwater systems. With the exception of Perth (1.5M population), all
major cities source their water from relatively soft surface waters. Perth is currently 60%
reliant on groundwater and 40% reliant on surface water sources. The need to soften their
water has therefore never been a major issue for most major cities.
It is more common for inland communities to utilise ground waters. Consequently it is in
these communities that waters of higher hardness are found.
4.2.2 Hardness Levels in Australian Cities and Towns
Table 4.1 shows the total hardness levels for the capital cities of each state. Soft waters are
found in Sydney, Melbourne, Hobart and Darwin. Slightly higher hardness levels are
found in Perth (groundwater impacts), Adelaide (end of 2500km Murray-Darling River
system) and Brisbane (nature of the catchment).
Table 4.1. Hardness Levels in Major Capital Cities in Australia
City
Population (million)
Sydney
Melbourne
Brisbane
Perth
Adelaide
Darwin
Hobart
4.2
3.6
1.7
1.4
1.1
0.1
0.2
Total Hardness Levels (as
mg/L CaCO3)
55
20
90
96
135
30
30
Outside capital city areas, hardness levels are more variable. A breakdown of hardness
levels in each of the states for these areas is shown in Table 4.2.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Table 4.2 - Australian communities receiving high hardness water
State
Nr. localities/ communities Population receiving
receiving hard water
hard water
(> 200mg/L CaCO3)
(> 200mg/L CaCO3)
Western Australia
South Australia
Northern Territory
Queensland
New South Wales
Victoria
29
27
9
20
42
20 (est)
Total
147
Proportion of Australians
receiving high hardness
waters
50,000
70,000
30,000
22,000
70,000
50,000 (est)
292,000
1.5%
Across Australia, around 150 communities are receiving water with hardness levels above
200 mg/L as CaCO3. Almost all these towns have hardness levels below 500mg/L with
the majority below 400 mg/L. Often water of these towns tends to be high in total
dissolved solids. However, water of this quality water only supplies 1.5% of the
Australian community.
These towns do not have any specific treatment for hardness removal. Based on a
population of approximately 300,000 using an estimated 300 m3 per annum per property,
estimated volumes of water supplied with high hardness levels are approximately 90 GL
per annum.
4.2.3 Technologies Currently used for Hardness Removal
With the exception of Perth, relatively few systems have hardness removal technologies
employed. For example, in NSW only 5 systems employ treatment for softening to bring
hardness levels to between 16 and 100 mg/L CaCO3. These treatment systems employ
standard lime-soda softening and service a population of around 75,000 people. In South
Australia and the Northern Territory, for towns with significant populations, no systems
utilise softening. In Queensland, treatment for hardness is also not widely used; however,
some communities blend surface and groundwater to reduce hardness levels. In Western
Australia, Calgon dosing is employed for small communities with high hardness levels.
Perth is the only major city in Australia where hardness levels require treatment. One
groundwater scheme with flow rates of approximately 100 ML/day (annual production
30GL) employs a lime crystallisation technique where small particles are introduced into
the treatment process and calcium carbonate builds up to form pellets which are then
removed. Hardness of the water is approximately 160 mg/L after treatment.
4.2.4 Reasons for softening in Australia
Where traditional lime soda or lime crystallisation is used for softening, the key benefit is
the reduction of scaling on household appliances and the production of water more
suitable for commercial and industrial process use. Aesthetic improvement of the water
also results (e.g. improved lathering).
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April 2007
Where reverse osmosis desalination technologies are intending to be employed (see
below), the above also applies, but the reduction in total dissolved salts is the key aim.
With reverse osmosis intending to be applied for wastewater treatment and polishing, the
major reason is the reduction of chemical toxicants and microbial pathogens.
4.2.5 Proposed Plans for treatment
The prolonged drought across many Australian areas is forcing significant numbers of
communities to rethink water supply strategies.
On the supply side, two key strategies are the implementation of reverse osmosis systems
to desalinate seawater and the potential reuse of wastewater for indirect potable reuse.
Desalination is possible as most major cities in the country are situated near coastal areas.
Brine discharges from these plants will be discharged via ocean outfalls subject to
environmental licensing constraints.
Reverse osmosis is also being considered as part of a treatment train to bring wastewater
quality to potable standards. However, community concern about these proposals is
apparent.
These treatment systems using reverse osmosis are not specifically targeting hardness
levels in water, but are being used as a potable supply augmentation strategy. Hardness
levels will, however, be lowered from seawater levels.
Final hardness levels from reverse osmosis plants will be adjusted based on investigations
on optimum levels in individual supply systems around the country. With Australians
currently very used to soft surface waters, it is highly unlikely that significantly harder
water will be introduced as a result of desalination augmentation. Blending of reverse
osmosis water with currently soft surface supply systems will likely be designed to ensure
hardness levels are somewhat similar to current levels as outlined in Table 4.1.
4.3 France
For France, not a complete overview on the extent of softening could be achieved. It is
estimated that for the whole of France less than 5% of produced drinking water is being
treated with central softening.
Veolia Water Company delivers to approximately 37% of the total French population
drinking water. They operate 21 softening plants, producing about 200,000 m3 water per
day. Figures 4.2 and 4.3 show the calcium and magnesium levels in water Veolia supplies
(corresponding to a population of 1.3 million inhabitants).
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April 2007
Percentage of population below a calcium concentration
100%
80%
60%
40%
20%
0%
0
20
40
60
80
100
120
Ca concentration (mg/L)
140
160
Figure 4.2. Calcium concentration distribution in population served by Veolia water
Percentage of population below a magnesium concentration
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0
5
10
15
20
25
30
35
Mg concentration (mg/L)
40
45
50
Figure 4.3. Magnesium concentration distribution in population served by Veolia water
Figure 4.4 shows drinking water hardness for the whole of France.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
55
Figure 4.4. Drinking water hardness in France (Internet, France, 2006)
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
4.4 Germany
In Germany, the hardness of untreated drinking water varies from below 0.5 (Black
Forest) up to 7 mmol/L (see Figure 4.5).
> 21°dH
21°dH
0°dH
0°dH
Figure 4.5. Drinking water hardness in Germany (Wasser.de, 2006)
In Germany, the aim of softening usually is to decrease hardness to a level below 2.5
mmol/L (class 2 of hardness). To minimize corrosion in some cases hardness is decreased
by mixing two water types.
In Germany, precipitation by soda and/or lime addition, CARIX, and membrane filtration
are used as softening techniques, depending on other water quality goals to be achieved
(e.g., nitrate removal, pesticide removal, etc.).
The minimum level of (carbonate) hardness after softening is recommended to be 1.5
mmol/L. However, this is not a regulatory requirement.
4.5 The Netherlands
In The Netherlands, hardness of raw untreated drinking water varies from 0.5 to 5
mmol/L. Currently about 40 softening plants are producing about 65,000 m3 water/hr.
This is approximately 50% of the total water production. The installed capacity is more
than this, as can be seen in figure 4.7, but not all capacity is currently being used.
The water produced usually has hardness levels ranging between 1 and 1.5 mmol/L.
In the Netherlands, there are no regulations for the minimum concentrations of individual
calcium and magnesium in water, but only for total hardness after softening/desalination
(1-2.5 mmol/L Ca + Mg).
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April 2007
Techniques that are applied for softening in the Netherlands are:
pellet reactors (mostly)
membrane filtration (at 11 locations)
reservoir softening (at 2 locations)
Figures 4.6 to 4.8 show the Dutch situation on softening and total hardness.
70
60
st 50
na
lp
fo 40
re 30
b
m
uN20
10
0
1970
1975
1980
1985
1990
1995
2000
2005
2010
2015
Figure 4.6. Number of softening plants in the Netherlands
120000
)h
/3 100000
m
( 80000
yit
ca
pa 60000
c
de 40000
ll
at
sn 20000
I
0
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015
Figure 4.7. Development of softening capacity in the Netherlands
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April 2007
Total Hardness drinking water (mmol/l)
4,5
4
red
= pellet softening
blue = NF/RO
3,5
yellow =other techniques
= = planned
green
3
l
2,5
2
Dutch policy
1,5
1
0,5
0
0
1
8
15
22
29
36
43
50
57
64
71
78
85
92
99
106 113 120 127 134 141 148 155 162 169 176 183 190 197 204 211
Figure 4.8. Drinking water hardness per station (data 2003). Black bars indicate stations without
softening. Coloured bars are stations with softening. In blue: planned softening plants with their
current hardness.
4.6 South Africa
Approximately 90% of South Africa's surface waters have hardness values of between 20
and 150 mg/L as CaCO3, (of which the majority is between 50 and 80 mg/l), with the rest
less than 20 mg/L. Only occasional boreholes in certain areas of the country have
hardness >250 mg/L. (Less than 10% of the country's potable water comes from
groundwater sources). Most of these boreholes are not used, or where they are used,
household water softeners are applied to soften the water.
4.7 Switzerland
In Switzerland no central softening is applied during drinking water treatment. Data
regarding the total hardness of drinking water in Switzerland are presented in Figure 4.9.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Figure 4.9. Hardness characteristics of drinking water in Switzerland (Internet, Switzerland, 2006).
(Yellow = 0-15°F, Orange = 15-25 °F, Red = > 25 °F)
4.8 United Kingdom
In the United Kingdom, no central softening is applied (British Water, pers. comm.).
Information regarding the extent to which point-of-use devices are used is not available.
Information from the British Drinking Water Inspectorate shows that drinking water in
the UK is generally considered to be 'very hard', with most areas of England, particularly
in the East, exhibiting above 200 mg/L as calcium carbonate equivalent. In areas such as
Wales, Cornwall and parts of North-West England the waters are softer and the
equivalent calcium carbonate levels range from 0 to 200 mg/L.
In Figure 4.10, the total hardness of drinking water in the UK is presented.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Figure 4.10. Drinking water hardness in the United Kingdom.
4.9 United States
Source water hardness in the United States (US) varies with regions and source water
types. Water from the eastern, northwestern, and Hawaii regions of the US are considered
softest (0-60 mg/L CaCO3). With some exceptions, the majority of raw waters in the midwest and southwestern US are considered hard to very hard (120 mg/L - 250 mg/L, up to
1100 mg/L). The majority of water supply in these regions is groundwater. The hardest
waters in the US (>1000 mg/L as CaCO3) (≈ 10 mmol/L) were measured in streams in the
southwestern US (Briggs and Ficke, 1977).
Figures 4.11 and 4.12 were compiled by the USGS for source water hardness in the United
States.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Figure 4.11. Raw water hardness groundwater sources in the US (Source: Internet, US Geological
Survey with minor modifications).
Figure 4.12. Number of stations with their hardness in the US (Source: Internet, US Geological
Survey with minor modifications).
4.9.1 Hardness removal
Although the US Environmental Protection Agency (USEPA or EPA) does not regulate
drinking water hardness, some utilities with elevated hardness concentrations in their
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
raw waters elect to include softening process at the treatment plants. Providing the public
with aesthetically acceptable level of hardness in treated water is the primary driver for
softening although it is recognized that changes in water quality parameters as a
consequence of lime/soda softening (e.g., increasing pH, alkalinity, DIC) would have
direct impact on corrosion (Dugan, 2006: personal communication).
Current water quality goals set by the American Water Works Association (AWWA),
which develops industry standards for products and processes for the drinking water
community in the US, do not include hardness. However, the acceptable design criteria
for softening facilities are recommended at 120 mg/L as CaCO3 or less for total hardness
and 40 mg/L as CaCO3 or less for magnesium hardness in finished water
(AWWA/ASCE, 1998).
More than 1,000 drinking water treatment plants in the United States include some form
of softening processes in their treatment trains. The majority of these softening plants is
located in the mid-western states and Florida, where groundwater with elevated hardness
concentrations is a major source of drinking water supply (AWWA/ASCE, 1998).
Lime/soda ash addition is the most often used treatment technique to soften water in the
US. This process is often applied in addition to coagulation, iron removal, or a
combination of coagulation and iron removal processes. Over 3,000 million gallons per
day of water is produced by facilities using such softening processes (AWWA/ASCE,
1998). This is estimated to be approximately 13% of the total drinking water production in
the US.
Membrane treatment processes such as ultrafiltration (UF), nanofiltration (NF), and
reverse osmosis (RO) are being increasingly applied for drinking water treatment in the
US due to advancement in technology that allows them to feasibly and effectively remove
multiple contaminants in the water. Softening is one of the multiple benefits recognized as
a result of membrane application. However, membrane softening is still regarded mostly
as a secondary benefit. With some exceptions, the application of membrane technology for
softening alone may not justify the high cost of installation, operation, pre- and posttreatment conditioning, and residual handling. However, as membrane technology
achieves greater longevity, lower cost and pretreatment would be better defined, the
opinion may change in the future.
Ion exchange (IX) processes is not widely applied water treatment plants for softening
purposes due to waste disposal concerns and also due to relatively high cost of operation.
The high salt content in the waste regenerate makes it difficult to handle and dispose.
However, a majority of in-home softening devices being used in the US are cation IX units
(Dugan, 2006).
4.9.2 AWWA’s survey of drinking water treatment in the US
In the 1996 AWWA’s survey of drinking water utilities, 443 groundwater systems (GWs)
and 547 surface water (and combination of surface and groundwater) systems (SWs)
responded. Based on the surveyed data, the average raw water hardness for SWs (122
mg/L as CaCO3 , n = 448) are generally lower than those from GWs (206 mg/L as CaCO3 ,
n = 378) (AWWA WaterStat, 1998). The ranges of raw water hardness were 0 to 1100
mg/L as CaCO3 for GWs and 2 to 1000 mg/L as CaCO3 for SWs (AWWA’s WaterStats,
1998).
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April 2007
Of the GWs responded to the survey, 68 plants included lime/soda ash addition; 11
plants included membrane processes; 15 systems included ion exchange; and two used
resin adsorption in the treatment train. Although it is recognised that the above
mentioned processes would affect hardness levels, the primary reason for using these
processes was not clear in the survey responses. The median values for average raw and
treated water hardness for GWs were 198 mg/L as CaCO3 (n = 378) and 137 mg L as
CaCO3 (n = 370), respectively. Treated water hardness levels for GWs ranged from 0 to
1100 mg/L as CaCO3 (AWWA’s WaterStats, 1998).
For systems that used surface or combination of surface and ground waters (SWs), 89
plants reported to have lime/soda ash addition, while only three systems included
membrane treatment (as RO or MF) in the treatment train. Ion exchange and resin
adsorption processes were reported only once for each. The median values for the plant’s
average raw and treated water hardness are 111 mg/L as CaCO3 (n = 440) and 108 mg/L
as CaCO3 (n = 490), respectively. Treated water hardness levels for SWs ranged from 0 to
867 mg/L as CaCO3 (AWWA’s WaterStats, 1998).
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April 2007
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April 2007
5 Reasons for conditioning
5.1 General
In chapter 3 several reasons for softening have been presented. However, it is not that soft
water has only advantages by definition. Groundwater can be naturally soft in situations
where no calcium carbonate is present in the subsoil (e.g., sandy soils). Due to the absence
of calcium carbonate, this water will not reach calcium carbonic acid equilibrium. Usually,
this water contains mainly carbon dioxide causing the water to be acidic. The same can be
true for desalinated water.
Water with a low pH usually has corrosive properties towards the piping material it is
transported by. The knowledge that water composition has a strong influence on
corrosion has resulted in the development and application of several water treatment
technologies. These technologies, often summarized as conditioning methods, include a
change in pH whether or not in combination with a change in calcium and bicarbonate
content. For additional information on conditioning techniques, see Annex III.
In addition to these technologies some countries use corrosion inhibitors, such as
phosphates and silicates, as means to control corrosion. Not every country allows the use
of corrosion inhibitors.
Different materials are used in drinking water distribution, each posing its own
requirements on the composition of drinking water as to avoid corrosion. An extensive
amount of information on conditioning and the effect on materials is available. In the
following paragraphs the most important aspects of the materials with respect to their
behaviour in contact with drinking water are described.
5.2 Asbestos cement/concrete
Asbestos cement and concrete are widely used to produce trunk mains and distribution
mains. These are both cement based products, cement being the binder for asbestos fibres,
gravel and sand. These materials are chemically rather inactive, which implies that
corrosion of cemented materials is restricted to the cement fraction.
During the manufacturing process water is added to the cement, resulting in hydration of
cement compounds (dicalcium silicate, tricalcium silicate, tricalcium aluminate and
gypsum) and the formation of cement stone ('hardening'). This also results in the
formation of free lime. In contact with water the hydrated cement salts and lime within
cement stone will dissolve. As a result the concentrations of calcium, aluminium, silica
and iron will increase. Due to the production of OH ions, the pH and alkalinity in the
pores will increase as well. Figure 5.1 gives a schematic structure of the cement based
material (Leroy , 1996)
-
et al.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Figure 5.1 Schematic structure of cement-based material (Leroy
et al.
, 1996)
If sufficient inorganic carbon is present in the water, diffusion of CO , HCO and CO
from the main water to the water in the pores will occur. Due to the high alkalinity in
these pores, a shift in the calcium carbonic acid equilibrium will take place and CaCO
will precipitate. As the volume of CaCO is larger than the volume of the compounds that
are in solution, the porosity of concrete will decrease and so will the corrosion speed (Van
Dijk & de Moel, 1983). In this way the precipitation of CaCO creates a protection layer in
the pipes.
In water with a very low total inorganic carbon content (TIC) the formation of CaCO3 will
not lead to blocking of the pores, even if the water transported is oversaturated with
CaCO (Schock and Buelow, 1981). In water of low TIC, the amount of CaCO
precipitation possibly may not be sufficient to protect the material if it is lower than the
lime solved. Hard waters of high TIC will potentially produce a greater mass of CaCO
precipitate than hard waters of lower TIC and will therefore be more protective.
With inadequate precipitation of calcium carbonate the water can dissolve a significant
quantity of lime. Consequently the material will be degraded. The lime dissolved into the
transported water also results in an increase of pH. In soft water calcium hydroxide
dissolves into the water at a rate that decreases over time with the depth of the attack. But
this process also depends on the evolution of the material’s porosity.
2
3
-
3
2-
3
3
3
3
3
3
Corrosion of concrete on the inside of the drinking water main may result in:
disintegration of the top layer of asbestos cement pipes and exposure of fibres that
might easily be released into the water;
an increased pH in drinking water and also an increase in suspended solids,
calcium, iron, aluminium and silicates;
a reduction in pipe strength and eventually fractures and leakage; and
increased energy loss in transport due to an increase in wall roughness.
Of all disadvantages the most emphasis is on fibre release. Although it has been
concluded that there are no adverse health effects caused by oral ingestion of asbestos
fibres (WHO, 2004), fibre release should still be avoided if only to limit the release of
asbestos fibres in the environment. According to Meyer (1982), corrosion of cement-based
materials and the release of asbestos fibres are within acceptable limits when the
Langelier Saturation Index is above -0.2 (SI> -0.2). Corrosion is obviously minimal when
the SI is greater than zero (>0). In addition to a sufficiently high SI, corrosion of asbestos
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
- 42 -
April 2007
cement pipes can also be reduced by the stimulation of the formation of calcium carbonate
protection layers.
5.3 Dutcile iron, steel and galvanised steel
5.3.1 Ductile iron and steel
Iron is the material most commonly used in water distribution systems. The corrosion of
iron is a complex process that involves the oxidation of the metal, ultimately to form a
precipitate of iron(III). Corrosion of ductile iron and steel pipes will rarely result in
structural failure, but can lead to water quality problems, such as discoloration and
microbial growth. Corrosion will result in the presence of free iron ions than can
subsequently interact with substances present in water and form a large number of iron
complexes on the pipe wall. Very bulky corrosion layers may result, causing an increased
hydraulic resistance in the pipe and a reduction in hydraulic capacity. Bulky corrosion
layers also have an adverse effect on water quality.
Various researchers have tried to model the most important processes involved in
corrosion layer formation. Most successful is the so-called 'siderite' model by Sontheimer
1981). In this model precipitation of Fe -compounds, especially of FeCO , plays an
important role in the development of protective corrosion layers. According to this model
a stable and firm corrosion layer only develops when formation of Fe containing
compounds occurs through FeCO and not through direct oxidation of Fe to Fe ions. In
case of the latter precipitation starts in the water itself and a powdery and porous
corrosion layer results.
Also Kuch
(1983) and Wagner and Kuch (1984) studied the relationship between
water composition and corrosion. They investigated the influence of various water quality
parameters and concluded that corrosion and iron release increase when:
pH decreases, especially at pH values below 7 to 7.5;
amount of organic matter decreases;
chloride and sulphate content increases;
buffering capacity decreases. The buffering capacity not only influences corrosion
itself but also the quality of the corrosion layer (measured as percentage of iron
built into the corrosion layer);
phosphate concentration decreases;
there are strong fluctuations in water composition.
2+
et al. (
3
3+
2+
3
3+
et al.
Similar findings were reported by Sarin
(2001), Sarin
(2004).
There is also practical experience with the application of phosphates in Germany and the
United States; water companies successfully dose phosphates to combat corrosion in
ductile iron pipes (Vik , 1996, Lytle
2003).
et al.
et al.
5.3.2
et al.
et al.,
Galvanised steel
Galvanised steel is one of the oldest and most common plumbing materials used to
transport drinking water. It consists of a steel pipe covered with a zinc top layer to protect
the steel from corrosion. The influence of water quality on the behaviour of the galvanised
pipe is related to the behaviour of the oxidised zinc layer. Main parameters influencing
the corrosion system are the pH and the flow velocity.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Rapid loss of zinc and subsequent tuberculation commonly occurs in soft acid waters that
have a pH below 7 (Rhodes Trussell , 1996). Under these conditions the life of the zinc
coating is only a few months. Once the zinc layer is dissolved, corrosion of the iron pipe
proceeds at a more rapid rate and tuberculation (formation of voluminous corrosion
products) begins to accelerate as well.
Effective water treatment controlling the corrosion of galvanised pipe involves the
formation of effective protective scales on the surface of the pipe. Generally, the most
effective alternatives found for controlling corrosion of galvanised pipe are pH
adjustment and the use of silicates or orthophosphates (Rhodes Trussell , 1996).
Galvanised steel pipe would give the best service at pH values between 7 and 8.5,
depending on the alkalinity of the water. Generally better service is achieved with hard
water.
et al.
et al.
5.4 Copper
Two types of corrosion are important for copper in water supply; general corrosion,
where the metal surface is uniformly attacked, and local corrosion, also called pitting
corrosion. Several factor can possibly influence the process of copper corrosion. It is not
meant to give a complete overview on this matter, but some major aspects will be
discussed below.
5.4.1 General corrosion
The extent to which general copper corrosion takes place is determined by the formation
of protective corrosion layers, residence time and water composition. General corrosion
starts off with a chemical reaction between copper and dissolved oxygen. On oxidation
copper dissolves and reacts with oxygen and other components to form a layer of
insoluble copper salts (cuprite layer or malachite layer). The solubility of protective layers
continues to decrease for years to decades, although the most dramatic decrease is usually
in the first several years (Edwards , 1996). Formation of protective copper layers does
usually not cause life span problems and reduces the copper release to drinking water
(Van den Hoven ., 1988; Ferguson
1996; Edwards , 1996).
A lot of research has been done after the mechanism of copper corrosion and the
compounds involved in the corrosion process. In general copper release increases with a
decrease of pH. Higher bicarbonate concentrations exacerbate copper corrosion rates and
by-product release (Van den Hoven , 1988; Edwards ,1996). At a constant pH,
copper release is a linear function of the bicarbonate concentration.
About the exact mechanism of general corrosion discussion is still ongoing (Merkel, 2001).
He compared the behaviour of copper in test water with reactions in copper pipes in
contact with drinking water. He concluded that copper corrosion is a complex nonequilibrium process driven by at least three chemical sub processes:
•
metal oxidation,
•
fixation of dissolved copper in the corrosion scale (malachite and cuprite) and,
•
solubility equilibrium of copper phases
He concluded that the source of water determined the copper scale phases to be formed.
Another conclusion of Merkel was that the formation of a malachite layer in a copper pipe
was not an effective protection against an oxygen attack, but that it served as a sink for
dissolved cupper from the pipe. He observed that during stagnation a malachite layer
et al.
et al
et al.,
et al.
et al.
et al.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
- 44 -
April 2007
could be formed which dissolved during non stagnant times leading to higher copper
levels in the water.
Copper release can also be influenced by the natural organic matter (NOM) concentration
in water (Efström Broo , 1998;Werner ,. 2003;). Werner showed that NOM
influences the formation and structure of the corrosion products. The observed copper
concentrations were very low in waters with low or very low concentrations of NOM.
Especially in waters with pH around 7.4 the influence of organic carbon on release and on
malachite formation could be verified well. It is not yet identified how NOM does
interfere with the copper corrosion system and which attribute of NOM is responsible for
the caused effect. The nature of NOM may be responsible for the different behavior in
respect to copper release.
et. al
et al
5.4.2 Pitting corrosion
Pitting corrosion in copper pipes is a serious form of corrosion, resulting in localized pits.
Already within a few months of installation of copper pipes, pitting corrosion can result
in complete perforation of the pipe wall. With respect to pitting corrosion in copper pipes
it is important to note that the conditions that cause a pit to start (inception) are different
from those that allow the pit to continue to develop (propagate). As a general guide, the
inception of pitting seems associated with the condition of the metal before it is exposed
to water, whereas propagation of the pit is a function of water quality. However, the
observation on inception of pitting does not seem to be in line with the current pitting
epidemics in the US in high pH, low carbonate water (pers. comm.. Schock, 2007).
Van den Hoven and van Eekeren (1988) concluded that pitting corrosion originates from a
very localised disturbance of the corrosive environment, e.g. an impurity in the metal or a
particle on the pipe wall. In weakly buffered water types, the water in the pit might
become very acidic which accelerates pitting corrosion. Therefore they concluded that the
water composition should be such that sufficient alkalinity is reached.
However, the role of alkalinity is less unambiguous as described by Van den Hoven and
Van Eekeren, and other factors may need to be considered as well. In a summary of
copper corrosion problems Edwards
(1994) described that pitting corrosion in copper
pipes occurred under several conditions:
in cold hard waters with a pH between 7 and 7.8, a high sulphate content relative
to chlorides and bicarbonate and high CO (Type I);
in hot water with pH below 7.2, a high sulphate content relative to chlorides and
bicarbonate (Type II); and
in soft water with pH above 8.0 (type III).
Edwards
(1994) also speculated that NOM may prevent copper corrosion by
preventing the formation of adverse scale types, because experience has demonstrated
that removal or alteration of NOM by certain treatment steps increases copper corrosion
problems.
et al.
2
et al.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
- 45 -
April 2007
5.5 Lead
When lead pipes corrode lead concentrations in water will rise, which is undesirable from
a health point of view. The concentrations of lead in water delivered by lead pipes are
predominantly determined by the same factors that determine general copper corrosion
(i.e. water composition, residence time and the presence of corrosion layers).
The relation between water composition and the saturation level (plumbosolvency) has
been studied both theoretically and experimentally (Van den Hoven, 1987; Schock, 1994;
Leroy, 1993; Schock , 1996). These studies show that the principal mechanism
involved in lead corrosion is the dissolution of the product of simple oxidation of the lead
metal, namely lead carbonate. Mainly pH, alkalinity and temperature govern the
solubility of the protective lead carbonate layer. Lead solubility decreases with increasing
pH, decreasing temperature and decreasing alkalinity. The solubility of lead increases
markedly when the pH is reduced below 8, because of the substantial decrease in the
equilibrium carbonate concentration. Thus, plumbosolvency tends to be at a maximum in
waters with a low pH and low alkalinity.
et al.
The effect of the dimensions (length and diameter) of lead pipes and stagnation in
domestic installations also influence the dissolution of lead. Experimental stagnation
curves show that the maximum lead concentration is reached after about 5 to 6 hours for
pipes of 10 mm internal diameter but only after several tens of hours for pipes of 50 mm
diameter (Kuch , 1983).
et al.
The role of oxidation-reduction potential, through the action of water disinfectants
(beyond dissolved oxygen), is also one of the governing criteria of lead corrosion. Under
high ORP conditions, a deposit of PbO can be formed, which results in much lower
plumbosolvency than the Pb(II) carbonates and orthophosphates. This was noted as early
as 1983 in some laboratory experiments, but is more widely discussed in (Schock
1996 . Extensive research currently under way at USEPA is showing that protection by
PbO rather than the Pb(II) solid species is much more common than was previously
believed. This mechanism seems to be especially pertinent to hard, high alkalinity waters,
whose lack of corrosivity towards lead is often incorrectly attributed to the hardness or
elevated carbonate content. Less-strong oxidants, such as oxygen and monochloramine,
will not produce PbO deposits on pipes. Orthophosphate is also believed to inhibit the
oxidation of Pb(II) carbonates to PbO (pers. comm. Schock, 2007).
2
et al.
)
2
2
2
5.6 Copper alloys
Brass and gunmetal are alloys of copper, zinc and a small percentage of lead. Brass and
gunmetal are often applied in compression fittings and in soldered fittings.
Lead release from copper alloys is generally dependent on the amount of lead in the alloy.
The more lead in the alloy, the greater amount of lead released from the alloy. Lytle and
Schock (1997) showed in their investigations that the highest amount of lead leached from
brass over the first two weeks of exposure, and lead levels dropped quickest during this
period. The pH influenced the time needed to “level off” the lead release by brass. At pH=
8.5 this time was shorter (60 to 70 days) than under more corrosive conditions at pH of 7.0
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
- 46 -
April 2007
(about half a year). Orthophosphate tends to act as an aging accelerator, which means that
the time needed to reach stable low lead levels is reduced.
Dezincification of brass will cause the material to become porous, and the occurrences of
fractures and leaks may result. Dezincification does not occur in alloys with zinc contents
less than 15%, such as gunmetal and red brasses (Oliphant, 2002). Dezincification will also
lead to an increase in zinc concentrations in drinking water. This has been confirmed by
field measurements, although the regulatory limit for zinc is rarely exceeded.
Practical experience with drinking waters has established that drinking water with pH
values below 7.6 will not initiate dezincification. In waters with pH values in the range 7.6
to 8.2 the corrosion rate is quite low taking typically 15 years to penetrate the wall of
fittings, even in hot water. However, when the pH of the drinking water is higher than
8.2, the corroded zinc is precipitated in the bore of the fitting as hydrozincite,
Zn (OH) (CO ) . The characteristic voluminous but hollow physical form of the deposit
gives this type of attack its name of “meringue” dezincification. The deposit produced
will seriously impede the flow of water through the fitting, typically within 3 years, and it
is this type of dezincification that gives rise to most frequent complaints (Oliphant, 2002)
If during lime softening pH is increased too much the water may be converted from a
quality that does not support dezincification to one that does by increasing the pH and
lowering the temporary hardness (Lytle and Schock, 1997).
5
6
3
2
5.7 Conditioning
As indicated in the previous paragraphs, corrosion of cement based and metal piping and
fitting materials is a complex process. Protection against corrosion and material release to
the drinking water in essence depends on the formation of protective layers at the inner
pipe surface. The formation, composition and morphology of these layers depend on
many parameters. However, the most important parameters that determine the corrosion
process are the oxygen content, the calcium content, the buffer capacity of the water, total
inorganic carbon (TIC) and the acidity (pH). In countries frequently adding chlorine or
other disinfectants disinfectant type and residual are very important as well.
Because oxygen must always be present in drinking water, corrosion cannot be controlled
by this parameter. Most parameters are related to the calcium carbonic acid equilibrium.
This means that controlling these parameters by water treatment technology – i.e.
conditioning – can be used as effective measure to control corrosion.
Another way of reducing the concentration of metals in drinking water is the addition of
corrosion inhibitors. In general chemicals containing orthophosphates and silicate can be
used. The corrosion inhibitor combine with ions in the water or on the inside of the pipe
to form salts that have limited solubility.
Orthophosphate has been used effectively to decrease copper release, especially in hard
water, and works by a combination of slowing of the oxidation of copper (most likely the
Cu to Cu step) and forming a thin, probably amorphous cupric orthophosphate film).
The behavior of orthophosphate using a solubility model based on hydrated Cu (PO )
has been used to successfully target and solve many copper release problems in high
alkalinity drinking waters in the US. While more refinement is needed for some details in
the model, especially at pH over 8, it successfully predicts both the trends and even
+
2+
3
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
- 47 -
April 2007
4
2
copper levels, in all but a few cases. Utilities can even use fairly simple jar tests to predict
behavior (Lytle and Schock, 2006).
In Germany, more and more water works nowadays are focusing on the possibility of
using phosphate, in order to handle the limit value for copper (Klinger, 2004). For the
evaluation of the efficiency of corrosion inhibitors regarding minimisation of copper
release test rig experiments were carried out in more than 20 water works throughout
Germany. The results demonstrate that orthophosphate containing corrosion inhibitors
influence the copper corrosion process. Optimisation of corrosion inhibitors in respect to
concentration and composition needs to be done in practice case by case for each water
quality.
Addition of orthophosphate has also shown to reduce the amount of lead that enters
drinking water from lead pipes. Calculations have shown that the addition of
orthophosphates can reduce the concentration of lead in drinking water that passes
through lead pipes to less than 10 µg/l. In practice, concentrations of between 10 and 30
µg/l have been achieved (Wagner, 1992). Formulations with silicate or only
polyphosphates give poor results for reducing lead concentrations (Schock , 1996;
Boireau , 1996).
et al.
et al.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
- 48 -
April 2007
6 Optimal composition of drinking water
The previous chapter has summarised and reviewed the knowledge on corrosion and its
dependence on water quality parameters. For each type of material guidelines can be
given for the water quality parameters that will minimise corrosion. Combining these
with the guidelines with respect to water hardness this results in guidelines for the
optimal drinking water composition. These hold true for water after desalination, water
after softening, as well as for naturally soft and naturally hard water. The fact that
drinking water has to be distributed through pipes necessitates the consideration of these
principles in order to avoid both corrosion of piping materials and scaling effects in
installations.
With this information, it becomes clear that recommendations on whether or not to soften
and minimum requirements for calcium, magnesium and total hardness should always be
seen within the context of the optimal composition of drinking water
Ideally the composition of water is such that scaling is prevented while at the same time
the interaction between water and piping material is as low as possible. Key aspects in
this optimal composition are maintaining the calcium carbonic acid equilibrium, a
sufficiently high pH and a sufficiently high, but not excessive, alkalinity. The water
companies in the Netherlands, for example, have developed a Code of Practice for water
leaving the treatment plant, based on these aspects. This is illustrated in Figure 6.1.
However, local situations and regulations will determine what will be an optimal water
quality. In the Netherlands no major disinfection with chlorine is applied and higher pH
values can be reached without concerns for disinfection by-product formation. But for
many North American water systems possibilities for pH adjustment to higher values are
limited when there are concerns for disinfection by-product formation. As a consequence
this will affect the feasibility and potential benefits of softening. In those situations other
measures to control corrosion have to be applied, such as applying corrosion inhibitors.
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
- 49 -
April 2007
Optimal conditions : - 0.2 < SI < 0.3
7.8 < pH < 8.3
TIC > 1.0 mmol/l
9,0
8,5
8,0
7,5
H
p
7,0
6,5
Optimal composition
of public drinking water
6,0
5,5
5,0
0,00
0,50
1,00
1,50
2,00
2,50
3,00
3,50
4,00
Ca [mmol/l]
Calcium carbonic acid equilibrium;
A guideline of -0.2 < SI < 0.3 is aimed for to avoid the water to become scaling, while at the same time
corrosion of asbestos cement is prevented
pH range
An upper and a lower pH limit are used by the water companies. The upper limit of 8.3 aims at prevention of
clogging caused by dezincification of brass and to prevent the formation of poor quality corrosion layers with
less protective effect. They apply two lower limits for pH; a fixed value of 7.8 and a variable value related to
TIC and sulphate content. The fixed pH value relates to plumbo solvency, the variable value relates to the
solubility of copper. Obviously, the higher of the two values determines the lower limit.
Alkalinity
To aid in the formation of protective calcium carbonate layers in pipe materials the water companies aim to
maintain a total inorganic carbon content (TIC) of > 1 mmol/L, preferably > 2 mmol/L.
Long time it was recommended to have a CI <1 to prevent iron corrosion. The corrosion index was defined
by ([Cl ] +[NO ] + 2[SO ]) / [HCO ]. The latest German guideline DIN EN 12502 does not give this
recommendation anymore, probably because to many other parameters influence the occurrence of iron
corrosion and in many cases the recommended value can only be reached by expensive technology to remove
chloride and sulphate.
-
3
24
3
Figure 6.1. Window for the optimal composition of drinking water in the Netherlands
([HCO ]=2[Ca ]).
3
2+
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
- 50 -
April 2007
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water. In: Internal corrosion of water distribution systems (second edition),
AWWARF/DVWG-TZW, AWWA Denver, p. 131.
Sontheimer, H., Kölle, W., Snoeyink, V.L., 1981. The siderite model of the formation of
corrosion-resistant scales. JAWWA 73, p. 572-579.
Vik E.A., Ryder, R.A., Wagner, I., Ferguson, J.F., 1996. Mitigation of corrosion effects. In:
Internal corrosion of water distribution systems (second edition), AWWARF/DVWGTZW, AWWA Denver, p. 430-433.
Wagner I., 1992. Internal corrosion in domestic drinking water installations, Journal Water SRT
- Aqua 41 (4), 219-223.
Wangnick, K., 2002. IDA Worldwide desalting plants inventory report no. 17, July 2002.
WDR 2006, Desal market to grow 12 % per year, Water Desalination Report , Vol. 42, No. 35
Werner W., Groß, H., Merkel, T., Detscher, E., Herzog, S., Eberle, S.H., 2002. ‘Influence of
inorganic compounds and natural organic matter on the release of copper in drinking water
installations’ Proceedings of Ceocor 2003, Sicily
WHO, 2006a. Nutrients in drinking water. World Health Organization, Geneva.
WHO, 2006b. Consensus Document of the Meeting of Experts on the Possible Protective Effects of
Hard Water Against Cardiovascular Disease. World Health Organization, Geneva.
WML, Watercompany Limburg 2005. Studie en beleid kalkafzettendheid. Investigation and
policy of scaling. Project number 89095-680150. (In Dutch).
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Annex I. Classes of hardness in GWRC
countries
Hardness classes for several GWRC countries are presented in tables I.1 to I.4.
Table I.1. Hardness classes in France
Very soft
mmol/L
0-0.0.5
Soft
Moderately
Hard
0.5-1
1-2
Hard
Very Hard
2-4
>4
Table I.2. Hardness classes in Germany
mmol/L
1
2
0-1.3
3
1.3-2.5
4
2.5-3.8
> 3.8
Table I.3 Hardness classes in The Netherlands
very soft
mmol/L
< 0.5
soft
fairly soft
0.5-1.0
1.0-1.8
fairly
hard
very hard
1.8-2.5
2.5-5.0
> 5.0
fairly
hard
very hard
3.2-4.2
> 4.2
hard
Table I.4 Hardness classes in Switzerland
very soft
mmol/L
< 0.7
soft
average
0.7-1.5
1.5-2.5
hard
2.5-3.2
United States
For the United States the concentration range for different classes of hardness in water
varies depending on which reference is cited. The United States Department of Interior
has established levels for classification of hardness in water, which is referenced by the
United States Geological Survey (USGS) and elsewhere.
Table I.5 The US Department of Interior’s classification for hardness (USGS)
US
1
mmol/L
Soft
0-0.17
Slightly
Moderately
Hard
Hard
0.17-0.6
0.6-1.2
Hard
Very Hard
1.2-1.8
>1.8
The American Water Works Association and the American Society of Civil Engineers’
Water Treatment Plant Design, 3rd ed. (1998) referenced Sawyer’s (1994) classes of hardness,
which is shown below in table I.6. A hardness concentration of approximately 120 mg/L
as CaCO3 is considered acceptable (AWWA/ASCE, 1998).
Table I.6 Hardness classes cited by the drinking water industry in the United States (Sawyer, 1994)
US
2
mmol/L
Soft
0-0.75
Moderately
Hard
0.75-1.5
Hard
Very Hard
1.5-3
>3
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Annex II. Other indicators for scaling potential
The relation between water composition and precipitation is more complex than generally
assumed. Drinking water hardness is an important parameter in this, but not the only one.
It has been observed that precipitation of calcium carbonate can also occur in very soft
water and waters that seem to have an almost identical composition can behave
differently with respect to the extent of precipitation.
In the Netherlands, other methods have been developed to measure the precipitation
potential of drinking water. These include:
- the analysis of the amount of precipitated calcium carbonate in practice (PACC)
- the calculation of the nucleation index, NI
- the calculation of the theoretical amount of calcium carbonate precipitation
(TACC90)
- the calculation of the saturation index at 90 °C (SI90)
- boiling experiment
The analysis of the PACC indicates the amount of precipitation that can occur under field
circumstances. The results of this analysis have shown a good relation with the scaling
effects in every day life (Brink et al., 1997). This method is applied to determine
quantitatively the scaling potential of drinking water.
The Nucleation Index is an indication of the extent to which acceleration of scaling can
occur by the solid compounds in water. This method is especially used for the
optimization of pellet reactors.
TACC90 and SI90 both indicate the super saturation of the water. These figures can be used
to predict the scaling effects if no sample is available to conduct a PACC analysis.
Based on the results of the PACC analysis, a good estimate can be made on the need for
softening (Brink et al., 1997):
PACC < 20 mg/l: less scaling
PACC between 20-60 mg/L: medium scaling
PACC > 60 mg/L: severe scaling
When no water sample is available to conduct a PACC analysis, a first indication can be
obtained from the TACC90:
TACC90 < 60 mg/l: less scaling
TACC90 between 60-120 mg/L: medium scaling
TACC90 > 120 mg/L: severe scaling
Although PACC gives a good view on the scaling potential of a water, the method has the
disadvantage that the experimental procedure is time consuming and expensive. This
means that only one measurement can be done per day. As an alternative, Water
Laboratory South has developed a new boiling test. In this test water is boiled for 5
minutes and the amount of precipitated calcium carbonate is measured by filtration. The
new method was validated in 2004. It was found that the results were reproducible and
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
show a very good correlation with PACC (Keltjens et al, 2004). The advantage is that the
boiling test can be done within approximately 1 hour.
Experience with TACC and boiling experiment
The Dutch Water Company Limburg (WML, 2005) investigated the total hardness,
TACC90 and the results of the boiling experiment of the water they distribute. At the same
time, a consumer questionnaire was conducted focusing on the number of descaling
activities (descaling of coffee machines, perlators in taps, water cookers) by the
consumers. Their results showed that the number of descaling activities was best
correlated with the results of the boiling experiment (see table II.1).
Table II.1. Results of the boiling experiment and the questionnaire (WML, 2005)
Considers Descaling Descaling Decsaling
Area
Boiling
experiment water to
of coffee of
of water
be hard
machines perlators cooker
(mmol/L
(%)
(n/yr)
(n/year)
(n/yr)
CaCO )
Extreme high value boiling experiment
1
Very high value boiling experiment
2
3
4
5
Rather high value boiling experiment
6
7
Regular value boiling experiment
8
9
10
11
12
13
14
12
2
1
1
Experienced
discomfort
(%)
**
3
1.62
92
16
11
15
80
1.4
80
13
11
16
71
0.9
83
15
11
16
68
0.75
49
9
6
11
42
0.7
88
9
8
13
69
0.57
64
6
4
8
48
0.55
59
6
6
10
45
0.43
43
4
5
6
20
0.4
48
6
5
5
32
0.4
38
3
1
3
10
0.38
53
4
4
5
22
0.3
40
6
4
9
25
0.21
26
7
4
9
20
12
24
15
5
20
3
2
3
2
2
4
1
0
1
1
0
2
1
2
3
2
1
3
0
0.2*
15
16
17
18
19
20
* = TACC90
0.2*
0.15
0.1
0.1*
0.05
0
4
6
12
5
2
10
3
** = experienced discomfort is expressed as the % of people experiencing rather much too extreme much
discomfort caused by scaling deposits
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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Annex III. Treatment technologies
Softening
Especially groundwater extracted from calcareous subsoils can have a high degree of
hardness, up to 6 or 7 mmol/L. Water extracted from deep sand layers on the other hand
are fairly soft. The hardness of surface water is normally at a level of about 2.5-3 mmol/L.
The hardness of water can be decreased by means of different processes. The choice for
the softening process will be dictated by the local circumstances (such as availability of
educated operators, size of the treatment plant, options for waste disposal), costs, the
composition of the water (TOC, calcium or magnesium hardness to be removed) and
other water quality goals to be reached, e.g. pesticide removal, nitrate removal.
Dosing of a base
Dosing of a base is a technique mainly applied during sludge softening and pellet
softening.
Usually, caustic soda (NaOH), lime (Ca(OH) ) or soda (Na CO ) are used as base. After
adding the base, a shift in the calcium carbonic acid equilibrium takes place. The
Langelier Saturation Index exceeds 1 resulting in crystallization of calcium carbonate. By
dosing the base in a reactor with seeding grains, crystallization will occur on the surface
of the seeding grains, forming limestone pellets. This process is called softening in a pellet
reactor.
2
2
3
When caustic soda is used the following reaction will occur:
NaOH + Ca + HCO
2+
3
-
CaCO + H O + Na
3
+
2
From these reactions, it can be concluded that when 1 mmol caustic soda is added, 1
mmol calcium and 1 mmol bicarbonate will be removed. At the same time the sodium
concentration is increased with 1 mmol and 1 mmol calcium carbonate is formed.
Lime
When lime is added, the following reaction will occur
Ca + 2 HCO + Ca(OH)
2+
3
-
2
→ 2 CaCO
3
+ 2H O
2
During this reaction, the sodium concentration is not increased. For each mmol lime
added, the calcium concentration is decreased by 1 mmol and the bicarbonate
concentration with 2 mmol. For a 1 mmol decrease in hardness, two mmol calcium
carbonate is formed (1 mmol calcium is added first in order to remove 2 mmol).
Drinking Water Hardness: Reasons and Criteria for Softening and Conditioning of Drinking Water
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April 2007
Soda
When soda is used as base, the following reaction will occur:
Ca + HCO + Na CO
2+
3
-
2
3
CaCO + HCO + 2 Na
3
3
+
-
During this reaction, the concentration bicarbonate remains unchanged, but for each
mmol decrease of hardness 2 mmol of sodium is added.
The choice of a base for use in softening is primarily determined by the composition of the
water to be softened. Because of different removal rates of hydrogen carbonate, the
relationship between the hydrogen carbonate content and total hardness is important
when choosing a base.
With the application of caustic soda 1 mmol/L HCO is used for the removal of 1
mmol/l calcium.
With lime 2 mmol/l HCO is used for the removal of 1 mmol/l calcium. If the
HCO concentration is low, softening with lime will not be possible, as this will
result in too much removal of HCO . This will have consequences for the
alkalinity of the water.
With lime twice the amount of calcium carbonate is formed than with the other
bases.
By dosing with soda no HCO is removed, but for each mmol/l calcium removed
2 mmol/l sodium is added. If the concentration sodium is already high in the raw
water, softening with soda will therefore not be the first option.
3
3
3
-
-
-
3
3
-
-
Ion exchange
Ion exchange involves the removal of the hardness ions calcium and magnesium, and
replacing them with non-hardness ions. The non-hardness ion is typically sodium
supplied by dissolved sodium chloride salt, or brine. The micro porous exchange resin
consists of sulphonated polystyrene beads that are supersaturated with sodium to cover
the bead surfaces. As water passes through this bed, calcium and magnesium ions attach
to the resin beads and the loosely held sodium is released from the resin into the water.
After softening a large quantity of hard water, the beads become saturated with calcium
and magnesium ions. When this occurs, the exchange resin must be regenerated, or
recharged. To regenerate, the ion exchange resin is flushed with a salt brine solution. The
sodium ions in the salt brine solution are exchanged with the calcium and magnesium
ions on the resin, and excess calcium and magnesium are flushed out with waste water.
Frequency of the regeneration or recharge cycle depends on the hardness of the water, the
amount of water treated and capacity of the resins.
In applying this process, 2 mmol/L of sodium is added to remove 1 mmol/L hardness.
CARIX process
The CARIX process (CARIX stands for carbon dioxide regenerated ion exchangers) is
based on the combined use of a weakly acidic exchanger and an anion exchanger in the
HCO form in a mixed bed. The exchangers remove both hardness and neutral salts from
waters. The CARIX process is applied in Germany especially if the removal of nitrate or
sulphate is needed. The exhausted exchangers resins are regenerated together by a
concentrated and pressurized carbon dioxide solution. As no other chemicals are used in
3
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April 2007
this process, the concentrate contains just the removed ions and is discharged without
treatment.
Membrane filtration
During membrane filtration, the water is processed under high pressure through a
membrane. Which membrane process is applied is mainly determined by the size of the
compounds to be removed (and other circumstances). For softening, mostly nanofiltration
and reversed osmosis are applied as other membrane processes use membranes with pore
sizes too large to remove hardness.
With increasing removal of compounds the risk of scaling at the membranes increases as
well. To avoid this acids or antiscaling chemicals (e.g. polyphosphates) have to be dosed.
As a consequence, the concentrate will contain sulphates and/or phosphates, making the
deposit of the concentrate more difficult.
When membrane filtration is applied as a drinking water treatment technique, the water
is conditioned to reach the optimum composition, before it distributed to the consumers.
Conditioning
There are several methods to increase the Saturation Index:
aeration
crushed limestone filtration
dosing of a base
Aeration
During aeration, the water is brought into contact with air. Water in equilibrium with air
will contain about 1 mg/L carbon dioxide, but groundwater will contain much more
carbon dioxide. When the water is brought into contact with air, the carbon dioxide will
disappear. Only the concentration of carbon dioxide will change; other parameters will
remain the same. This way of deacidification can be applied when the hydrogen
carbonate level is already high enough for a sufficient alkalinity. In that situation aeration
will be the best treatment technique as this will result in the largest increase in pH.
Crushed limestone filtration
This can be done by aeration of the water which results in a removal of carbon dioxide.
The SI will then become high enough, but the pH and HCO content will still be too low.
Therefore it is preferred to apply crushes limestone filtration. The aggressive water will
dissolve the limestone grains and carbon dioxide will be removed according to the
following reaction equation:
3
CaCO + CO + H O Ca + 2 HCO
3
2
2
2+
3
-
-
By applying crushed limestone filtration, the hardness of water and bicarbonate level will
increase. It is therefore also applied for water with low bicarbonate levels.
Dosing a base
Another way of neutralizing carbon dioxide is by adding a base to the water. There are
several bases that can be used for this: caustic soda, lime or soda.
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April 2007
The carbon dioxide is then converted to hydrogen carbonate according to the following
reaction equations:
CO + NaOH Na + HCO (caustic soda)
+
2
2 CO + Ca(OH)
2
2
3
Ca + 2 HCO (lime)
2+
CO + H O + Na CO
2
2
2
-
3
3
-
2 Na + 2 HCO (soda)
+
3
-
Dosing the base is a precise process and the amount dosed should be corresponding to the
amount of carbon dioxide to be removed. When insufficient amount is dosed the water
will remain acid; if too much is dosed, super saturation will take place and CaCO
precipitation will occur (softening).
This process is therefore usually only applied for smaller pH-corrections.
3
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